Devices for examining the optical properties and surface conditions of samples are known in the art. Generally, measuring devices use a light source which directs light at the surface to be examined and a detector detects this light and evaluates the conditions of the surface based in part on an analysis of the light that interacts with the surface.
The appearance of an object is composed of many elements such as gloss, distinctness of image (DOI), and haze Gloss is an important quality criterion for assessing the quality of paints, coatings, plastic surfaces and the like. Measuring gloss with results that are repeatable and precise is, however, exceptionally difficult. In its general definition, gloss is the property of a surface regarding its ability to reflect light. With high gloss surfaces, the angle of reflection equals the angle of incidence of the incoming light. Thus, the light reflected off the surface is reflected along the same angle as the incoming light, on the opposite side of the perpendicular ray from the surface. However, the more complex the shape, the more difficult it is to accurately measure gloss. Considerable physical deviations within a sample make it difficult to standardize the results of gloss measurements. Thus, due to surface conditions of the sample, the specular angle of the light reflected off the surface can change.
The appearance of an object is comprised of other factors beyond gloss, such as distinctness of image (DOI) and haze When the surface of a sample is not perfectly smooth, the incident light reflected off that surface scatters in a number of different directions from the specular angle. Distinctness of image (DOI) is the measure of this spread in the specular reflection due to scattering by fine surface structures of the sample. Reflection haze refers to a cloudy or milky appearance, also due to scattering of light, and it is defined as the spread of the specular component of the reflected light from a glossy surface.
Gloss meters, such as that described in commonly owned, co-pending application Ser. No. 13/327,072 filed on Dec. 15, 2011, hereby incorporated by reference, are configured to measure and display the results of a technical analysis of the gloss characteristics of a surface.
However, standard gloss meters are not configured to have integrated DOI and haze meters. For example, many of these prior art devices have technical limitations due to component selection and orientation which prevent their modification into DOI meters.
In a typical gloss meter, such as Datacolor's 45G® spectrophotometer, a beam of light is sent onto a sample at a fixed angle (e.g. 60 degrees). A pick-up optical channel is put at the same angle on the opposite side of the sample. Light coming from the sample at the same angle is focused through a pick-up lens and is delivered to a sensor.
However, most gloss meters measure the reflected light at a position of ± several degrees from the specular angle. Therefore, these devices cannot receive and evaluate the amount by which the light spreads from the specular angle due to an irregular surface. The light that is spread 0.3 degrees from the specular is responsible for (DOI). In contrast, the light that is spread farther from the specular angle, for example at 2 degrees from the specular angle, is responsible for narrow-angle haze. The light that is spread farther, for example, at 5 degrees from the specular angle is responsible for wide-angle haze. Thus the prior art devices are limited in their ability to determine true DOI and differentiate that characteristic from the haze characteristic.
The article entitled Standard Test Methods for Instrumental Measurement of Distinctness-of-Image Gloss of Coating Surfaces, ASTM D5767-95(2004) hereby incorporated by reference, describes traditional methods of measuring DOI. For example, a device with a narrow aperture for the light source and the detector is used to make measurements at the specular angle and slightly off the specular angle (±0.3 degrees). In an alternative method, light is passed through a narrow slit and is projected onto a specimen, and the reflected image intensity is measured through a sliding comb filter to provide a value of image clarity related to DOI. In a further alternative method, a pattern is projected onto the specimen and the reflected image intensity is measured to provide a value of image clarity. Furthermore, the standard measures and methods to measure reflection haze are known in the art. For example, in one arrangement, the gloss reflectance factor is measured at 30 degrees to the specimen normal, DOI is measured at ±0.3 degrees from the specular direction, and reflection haze is measured at ±0.2 degrees from the specular direction for a narrow-angle, and at ±5 degrees from the specular direction for a wide angle.
Different prior art references have described using these prior art methods to measure DOI. For instance, U.S. Pat. No. 4,746,805, hereby incorporated by reference provides a single meter to measure gloss and DOI of a painted surface. However, this disclosure requires the use of shutters and other mechanical devices that increase the complexity of the meter. What is needed is a DOI meter that has a simplified means of operation and construction.
Likewise, U.S. Pat. No. 5,155,558 to Tannenbaum, herein incorporated by reference, describes a method and apparatus for analyzing the appearance features of a surface using a scanning imaging detector translatable through a sequence focal plane positioned along the optical axis. Tannenbaum describes conventional image analysis using a scanned imagining detector. For example, Tannenbaum describes using Fourier Transformations to convert the scanned pixel intensities into spatial frequency domain datasets. One drawback to this system is that the device is complex in both operation and manufacture. Furthermore, it fails to determine the surface conditions of the sample through analysis of the scatter from the divergence of angles based on focusing the light on the sensor.
As such, what is needed in the art is a mechanism for simplifying and streamlining the generation of DOI, gloss and haze measurements where the light sensor is placed at the focal point of a lens, a collimated beam of light is directed at the lens, and surface characteristics are determined based on the amount of light scattered away from the focal point.